Breeding strategy for the Lower North of Thailand has set a goal for cultivars with durable resistance to rice blast since the causal agent (Pyricularia grisea Sacc.) possesses a great diversity for both genetic and pathotype. Pathotype is the pattern of response of different resistant genes to the tested pathogen in terms of compatible (susceptible) and incompatible (resistant) reactions using a differential set. Effective blast resistant genes for the Lower North of Thailand was continuously evaluated since 2003 through the IRRI' s 18 near isogenic lines (NILs) having 1 gene in each line. In 2007/2008, eighty isolates of P. grisea were collected from rice varieties having diverse genetic background from irrigated, rainfed, local to upland origins grown in the blast nursery and farmers' fields in Phitsanulok and the other Lower North provinces nearby. Pure cultures of the pathogen were obtained through the single spore isolation method and pathotyping was performed by testing all 80 isolates against NILs. There were 13 pathotypes characterized at 80 percent similarity. The pathogenic isolates tested were very aggressive since the most effective resistant genes showed only 25-33 percent degree of resistance. These genes were Pi 4a(t) (33.33 percent), Pi ta sup(2) (31.15 percent), Pi 1 from IRBL1-CL (29.03 percent) and Pi k, Pi k-p, Pi k-h (25.00 percent). Among these, Pi k-p was the most stable broad resistant gene during 2003-2008.